The unique optical properties of metals are at the core of many areas of research and applications, including plasmonics, metamaterials, superlensing and sub-diffraction focusing, optical antennas, and surface enhanced Raman scattering. One important length scale inherent in metamaterials and plasmonics research activities in the microwave, terahertz, infrared, visible, and ultraviolet ranges is the skin depth of metal, which remains at the sub-micrometer level throughout the broad spectral range.
One prominent question is whether we may be able to control terahertz electromagnetic waves (hereinafter, referred to as terahertz waves) down to the nanoscale, to achieve new functionality in the sub-skin depth regime.
In general, extraordinary transmission at a metallic film with a structure of periodic aperture arrays by surface plasmons has been studied in depth in the visible range. In such structure, incident light is effectively transmitted through an aperture considerably smaller than a wavelength. In recent, the studies for the transmission property have been widened to the infrared, terahertz, and microwave ranges. Transmission resonances in these ranges are known to be related not only to a surface wave on a metallic film but also to a variety of phenomena according to the aperture structure.
The terahertz waves exhibit the transmission resonance similar to the phenomenon that has been studied in the visible range, but the principle is much different. However, understanding for the principle of the transmission resonance in the terahertz waves has been insufficient so far. The reason for recent active studies for the terahertz waves is because wavelengths of light not harmful to human beings and emitted from various cells in a human body are present in the terahertz wave range, which results in increase in applications such as medical equipments or security and monitoring systems.